The production of nitriles by ammoxidation of an appropriate hydrocarbon in the presence of a suitable catalyst is well known. The production of acrylonitrile, for example, from a gaseous feed of propylene, ammonia and air is described by Bruce E. Gates et al in Chemistry of Catalytic Processes, McGraw-Hill (1979), pp. 380-384.
The feed is sent to an ammoxidation reactor where, in the presence of a suitable catalyst, acrylonitrile is produced along with lesser amounts of other nitrogen-containing compounds. The effluent from the ammoxidation reaction is quenched with water and the desired products are obtained in the liquid phase. The gas phase by- products, typically oxygen, carbon dioxide, carbon monoxide and unreacted hydrocarbon, can be combined with natural gas and sent to a boiler for combustion as disclosed, for example, in Yoshino et al., U.S. Pat. No. 3,591,620.
Callahan et al., U.S. Pat. No. 4,335,056, discloses the catalytic ammoxidation of propylene to produce acrylonitrile, followed by the catalytic conversion of propylene in the waste stream to acrylonitrile.
More recently, Khoobiar et al., in U.S. Pat. No. 4,609,502 disclosed a cyclic process for producing acrylonitrile using propane as a starting material. The propane is initially catalytically dehydrogenated in the presence of steam to form propylene. After ammoxidation, the effluent is quenched and the desired product removed, and the off-gases, including propylene and propane, are sent to an oxidation reactor to remove oxygen by selective reaction with hydrogen to form water vapor. The gas mixture exiting the selective oxidation reactor, which contains unreacted propane and propylene, light hydrocarbons and carbon oxides, is treated is in a separator to remove some of the light hydrocarbons and carbon oxides from the gas stream, The propane- and propylene-containing gas stream is then is recycled to the dehydrogenator. A disadvantage of the Koobier et al. process is that it is necessary to remove oxygen from the recycle stream, otherwise the oxygen would degrade the dehydrogenation catalyst.
Ramachandran et al., U.S. Pat. No. 4,868,330, discloses a recycle process for the ammoxidation of propylene with oxygen and ammonia in which, following ammoxidation, the product is removed by quenching, the gas stream is compressed and sent to a separator, such as a pressure swing adsorption (PSA) unit, to remove carbon oxides and propane, and the remaining gas stream, which contains propylene and propane, is recycled to the feed stream. In the process of this patent, propane, which is unreactive, is introduced into the system as a flame suppressor. Propane is a convenient choice for use as a flame suppressor since commercial grade propylene contains propane as an impurity. A disadvantage of the Ramachandran et al process, however is that, since the propane is not reactive in the ammoxidation reactor, it is necessary to continuously remove a portion of the propane entering the system to prevent its buildup therein.
Ramachandran et al. U.S. Pat. Nos. 4,849,537 and 4,849,538 disclose a recycle process for the production of nitriles comprising dehydrogenation of propane to propylene; ammoxidation of the propylene with oxygen and ammonia to produce a product gas stream containing the nitrile product, byproduct carbon oxides, and unreacted propane, propylene and oxygen; removal of the nitrile from the product gas stream by quenching; removal of some of the carbon oxides from the nitrile-free gas stream by PSA; removal of oxygen from the gas stream; and recycle of the resulting propane- and propylene-rich gas stream to the dehydrogenator. In U.S. Pat. No. 4,849,537 a multistage reactor is used to dehydrogenate the propane, and propylene product is withdrawn from a reactor stage intermediate the first and last reactor stages while unreacted oxygen is removed by passage of the product gas stream through a reactor stage downstream from the stage from which the propylene is withdrawn. In U.S. Pat. No. 4,849,538, oxygen is removed from the product stream by means of a selective oxidation reactor located between the PSA unit and the dehydrogenator.
The latter two Ramachandran et al. patents provide useful procedures for recovering unreacted propane from the waste product gas streams produced in propane dehydrogenation/ammoxidation plants, thereby avoiding the wasteful use of the propane as fuel. It would be advantageous to provide an efficient process and system for the conversion to ethylenically unsaturated nitriles of alkane impurities contained in a hydrocarbon feed stream to an alkene ammoxidation reactor. The present invention accomplishes this result and thus permits the use of an alkene feed stream containing an alkane impurity as feed for an alkene amoxidation reactor.